There are many instances in which it is disirable to laminate a transparent or partially transparent protective film onto a data-bearing substrate. For example, paper menus that are protected with laminated protective films are often improved in initial appearance and remain usable for much longer periods than those which do not have such protection. Documents and photographs can be preserved for historical or sentimental purposes much longer than would otherwise be possible if covered with a protective transparent film covering laminated to them. Even plastic articles, such as identification cards, signs, and the like, can be materially increased in longevity by laminating a tough, abrasion resistant transparent film onto their surfaces. All such articles are referred to generally, in this specification, as "documents".
One commonly used transparent protective film is of composite construction, having one surface formed of a tough, abrasion resistant polyester resin, the opposite surface comprising a material that serves as an adhesive for bonding the film to a substrate. The most common adhesive material is polyethylene. These composite polyester-polyethylene films have been successfully used for the protection of a wide variety of articles, including photographs, menus, identification cards, directional signs, and others.
One prevalent problem in laminating transparent protective films to various objects, where the bonding surface is one which requires heating to obtain good adherence, as in the case of polyethylene, is the maintenance of effective control of the heating of the adhesive surface of the film. With insufficient heating, adherence is likely to be inadequate, and the film may tend to peel away from the substrate that it is supposed to protect. On the other hand, if the polyethylene or other adhesive material on the inner bonding surface of the film is heated too much, it may tend to flow when pressure is applied to laminate the film to a substrate, fouling the laminating apparatus. Control of the heating of the adhesive surface of the protective film is thus important to the laminating process and apparatus, particularly in high volume operations.
Another substantial problem is presented in those instances in which the substrate is relatively flexible, as is frequently the case. The heating of the protective film immediately prior to pressure lamination tends to expand the film as the laminating process proceeds. When the film subsequently cools, its contraction may cause the completed laminate to curl upwardly on the side to which the film has been applied. This problem is particularly noticeable in laminated articles having a protective film on only one side, but may even occur where the film is applied to both sides of a substrate, due to minor variations in the film thickness, differences in the heating of the two laminating films applied to opposite sides of the substrate, and other factors. The resulting curl in the laminated articles, which may occur in a transverse direction as well as in a longitudinal direction, is frequently quite objectionable.